If you like to listen to music in the car using your iPhone, you'll know how frustrating it is locating your fingers to the right part of the screen to simply pause/play or skip a song. It's why I think music controls in a car should always use actual buttons, it's easy to know where the buttons are and what they do without looking. But gestures is another way to fix the problem.
CarTunes is a perfect app for the person who loves listening to their tunes while driving. The main interface (above) is what you'll be using when you're driving and the controls are very simple. You tap to pause/play and swipe left or right to go to the next/prev song. To adjust volume, you hold down on the screen and then swipe up or down. When changing volume you're shown a very cool colour changing indicator which overlays the screen - when the volume is low it is green, when the volume is high it is red. It then becomes easy to see how high the volume is at a glance.
By swiping up you're given the usual interface where you can choose songs, albums and playlists.
There is an iPad version too but I don't think it's a good idea mounting your iPad in your car if it obstructs your field of view.
Like I said earlier, it's perfect for those who listen to tunes while driving.